It is often desirable to display two or more separate video sources on one display simultaneously. Picture-In-Picture (PIP) is one such format in which two video sources are displayed, one appearing to be on top of the other. Display devices typically function in a raster mode in which the pixels of the display device are rendered in sequential order starting with the top left pixel, continuing to the right along the first line, then dropping down to the left side of the second line, continuing along the second line, and so on to the bottom line of pixels to complete one full video frame. The rasterization process repeats typically at a rate of 30 or 60 frames per second.
Rendering multiple video sources is inherently incompatible with rasterization process of a display device, as the display device is incapable of rendering just a single video source on a portion of the display. As explained above, the display rasterizes across all pixels of the display 30 times per second, and not just a subset of the pixels.